Re: If crossing over were to occur only in one sex, which sex should that be?

That sex would not necessarily be better than the other, it would just be 
more diverse as far as its contribution to any child.  Let's say for a 
minute that the sperm did not undergo crossing over but the egg did (not 
to say that men are inferior to women in any way or that the diversity of 
their sperm is not of equal importance). If this were the case, then all 
of the sperm would have the exact same genes in them, there would be no 
diversity among them. The female and her eggs would bear the sole 
responsibility in producing children that were different.  Since crossing 
over helps to make every little sperm and egg just a little different and 
unique, if one sex did not do this then that individual's sperm or egg
would not be as unique.  Actually it would be pretty much the same, with
the exception of course of the difference between those sperm carrying an x-
chromosome and a y, and the eggs with the different x-chromosomes since 
females have two different ones.  Let's say that neither sex underwent 
crossing over during meiosis.  If two individuals of this nature were to 
have children, say four boys (I say boys because any girls they had would 
be more diverse than the boys because during the very early stages of 
development one of the x-chromosomes will shut off in each cell, so the 
daughters would still be a little different from each other; this would 
effect boys more than girls), odds are that two would look a lot like each 
other, I mean a lot... same for the other two, because which x-chromosome 
they got from their mother would be the only difference between them.

   Now for the rat the human and the sea urchin: yes, their sperm is 
different.  On a genetic level it is very different because one is for a 
rat one for a human and the other for a sea urchin.  They also look a 
little different as well. The sea urchin uses external fertilization, it 
releases every thing into the water.  Because of this they do not have to 
worry about some of the things that the other two do.  Mammalian sperm on 
the other hand must survive in the female reproductive tract before it 
reaches the egg. The human and rat sperm are probably closer to each other 
simply because they are both mammals.  Whew... I hope that's enough.

Moderator's Note:
That's not the whole story, as Steve Mack adds:

"It seems like this is overlooking the independent assortment of diploid
chromosomes during the formation of gametes. Since every individual
has 2 chromosomes for which they are likely heterozygous, even
without crossover events, their gametes would still differ
considerably, not just at the sex chromosomes as is suggested above.

Also, it seems like sea-urchin sperm would have more to 'worry' about
than mammalian sperm -- they are predated on by several species
including some bi-valves (e.g. pest species like Corbicula fluminea, the
Asiatic Clam or various Dreissena species)."

With 23 pairs of human chromosomes, there are over 8 million possible
haploid states even in the absence of crossing over.